It is known that a latent electrostatic image can be developed with toner particles dispersed in an insulating nonpolar liquid. Such dispersed materials are known as liquid toners or liquid developers. A latent electrostatic image may be produced by providing a photoconductive layer with a uniform electrostatic charge and subsequently discharging the electrostatic charge by exposing it to a modulated beam of radiant energy. Other methods are known for forming latent electrostatic images. For example, one method is providing a carrier with a dielectric surface and transferring a preformed electrostatic charge to the surface. Useful liquid toners comprise a thermoplastic resin and dispersant nonpolar liquid. Generally a suitable colorant is present such as a dye or pigment. The colored toner particles are dispersed in the nonpolar liquid which generally has a high-volume resistivity in excess of 10.sup.9 ohm centimeters, a low dielectric constant below 3.0 and a high vapor pressure. The toner particles are less than 30 .mu.m average particle size as measured using the Malvern 3600E Particle Sizer described below. After the latent electrostatic image has been formed, the image is developed by the colored toner particles dispersed in said dispersant nonpolar liquid and the image may subsequently be transferred to a carrier sheet.
Since the formation of proper images depends on the differences of the charge between the liquid developer and the latent electrostatic image to be developed, it has been found desirable to add a charge director compound and preferably adjuvants, e.g., polyhydroxy compounds, aminoalcohols, polybutylene succinimide, an aromatic hydrocarbon, metal soaps, etc. to the liquid developer comprising the thermoplastic resin, dispersant nonpolar liquid and preferably a colorant. Such liquid developers provide images of good resolution, but it has been found that charging and image quality are particularly pigment dependent. Some formulations, suffer from poor image quality manifested by low resolution, poor transfer efficiency and poor solid area coverage. Solid area coverage is often reduced by crater-like defects (mottle) formed during image fusion. In order to overcome such problems much research effort has been expended to develop new type charge directors and/or charging adjuvant for electrostatic liquid toners.
It has been found that the above disadvantages can be overcome and improved developers prepared containing a dispersant nonpolar liquid, ionic or zwitterionic charge director compound, a thermoplastic resin, a colorant and an adjuvant compound described more fully below. Depending on the charge director used the developers containing these salts may be positive or negative. The improved electrostatic liquid developer when used to develop an electrostatic image results in improved image quality, transfer efficiency, and solid area coverage, and reduced mottle independent of the pigment and charge director present.